tests/test-diff-binary-file

prepush: add more precise error messages

Part of the patch is from [e-mail]

- indicate the branch name where there are multiple heads
- give better advice when hitting a possible race, where new heads are added
  between discovery and the call to branchmap(). In that case, asking the user
  to merge isn't helpful, since only remote has the changes.

#!/bin/sh

hg init a
cd a
cp $TESTDIR/binfile.bin .
hg add binfile.bin
hg ci -m 'add binfile.bin'

echo >> binfile.bin
hg ci -m 'change binfile.bin'

hg revert -r 0 binfile.bin
hg ci -m 'revert binfile.bin'

echo % diff -r 0 -r 1
hg diff --nodates -r 0 -r 1

echo % diff -r 0 -r 2
hg diff --nodates -r 0 -r 2

echo % diff --git -r 0 -r 1
hg diff --git -r 0 -r 1

echo % diff --git -r 0 -r 2
hg diff --git -r 0 -r 2